The disclosure relates generally to free-space optical (FSO) terminals, and more particularly, to a simplified FSO terminal architecture.
Conventionally, two monostatic FSO terminals in conjunction with two corresponding optical modems establish and utilize a data link to send and receive optical data signals. Due to a high directionality of the data link, very high precision beam steering is required. Thus, the two monostatic FSO terminals include and utilize some sort of beam steering element (e.g., a tip/tilt mirror) to actively point, send, and receive, therebetween, and the optical data signals generated by the corresponding optical modems. For closed loop tracking, portions of those optical data signals are used for position information to achieve an optical alignment for the data link, while data of these signals is parsed and processed. In this regard, the two monostatic FSO terminals utilize separate quadrant/position sensing detectors to track the incoming optical data signal and generate an error signal for controlling the beam steering element.
For example, a first optical modem generates and provides an outgoing optical data signal to an optical fiber of a first monostatic FSO device. The optical fiber directs the outgoing optical data signal to a first beam steering element of the first monostatic FSO device, which projects the outgoing optical data signal as a beam to a second monostatic FSO device. The second monostatic FSO device receives the beam, as an incoming optical data signal, through its aperture. In conventional monostatic FSO terminals, an additional, second beam steering element is needed in the second monostatic FSO device (in this example) to direct, through a passive beam splitter, a portion of the beam to a quadrant/position sensing detector in the second monostatic FSO device. The quadrant/position sensing detector provides position information to a controller of the second monostatic FSO device that adjusts the first beam steering element as needed to achieve an optical alignment between the first and second monostatic FSO terminals. A remainder of the beam, which includes the data, is received and passed by an optical fiber of the second monostatic FSO device to a second optical modem for processing. Note that, at the same time, the second monostatic FSO device is also sending an outgoing optical data signal that is received and processed by the first monostatic FSO device in a similar manner.
Optical alignment between the two monostatic FSO terminals is a key consideration that introduces significant complexity and cost to the design of these monostatic FSO terminals. In particular, any drift in a relative optical axis between the optical fibers and the quadrant/position sensing detectors can result in highly degraded acquisition and tracking of the optical data signals. Further, any significant misalignment before an initial acquisition of the optical alignment could prevent ever acquiring the data link between the monostatic FSO terminals.
Additionally, the quadrant/position sensing detectors include at least three (e.g., four) individual detectors and a common cathode. The common cathode is shared by and, in turn, sets a noise floor for the at least four individual detectors. Thus, the common cathode limits the noise floor to higher levels, which furthers limits the acquisition and tracking link margin for the monostatic FSO terminals.
Thus, there is a need for an improved FSO device/system that overcomes at least these deficiencies of conventional FSO terminals/systems.